Rapid Detection of Staphylococcus aureus Strains with Reduced

Methicillin-resistant Staphylococcus aureus (MRSA) usually harbors a vancomycin-susceptible phenotype (VSSA) but can ex-hibit reduced vancomycin susceptibility phenotypes that can be heterogeneous-intermediate (hVISA), intermediate (VISA), or fully resistant (VRSA). Current detection techniques (e.g., Etest and population analysis profiles [PAPs]) are slow and time-con-suming. We investigated the potential of microcalorimetry to detect reduced susceptibilities to vancomycin inMRSA strains. Representative MSSA, VSSA, hVISA, VISA, and VRSA reference strains, as well as clinical isolates, were used. PAPs were per-formed by standard methods. Microcalorimetry was performed by inoculating 5 107 CFU of overnight cultures into 3-ml vials of brain heart infusion broth supplemented with increasing concentrations of vancomycin, and growth-related heat production was measured at 37°C. For the reference strains, no heat production was detected in the VSSA isolates at vancomycin concentra-tions of>3g/ml during the 72 h of incubation. The hVISA and VISA strains showed heat production with concentration-pro-portional delays of up to 6g/ml in 48 h and up to 12g/ml in 72 h, respectively. The VRSA strain showed heat production at concentrations up to 16g/ml in 12 h. The testing of clinical strains indicated an excellent negative predictive value, allowing us to rule out a decreased vancomycin susceptibility phenotype in<8 h of incubation. Sequential isolates from a patient undergo-ing vancomycin therapy showed evolving microcalorimetric profiles up to a VISA phenotype. Microcalorimetry was able to de-tect strains with reduced susceptibilities to vancomycin in<8 h. The measurement of bacterial heat productionmight represent a simple and rapid method for the detection of reduced susceptibilities to vancomycin inMRSA strains